Epson PX-8 Technical Manual page 41

REV.-A
2.2.3 Low Voltage Detection
The 4-bit sub-CPU always monitors the battery voltage through an AD converter (uPD7001).
When the battery voltage falls to +4.7V or below, the main battery is switched to the auxiliary bat-
tery.
2.2.3.1 Battery Voltage Detection Circuit
This circuit monitors the main battery voltage output through fuse F1 as follows:
• The built-in program of the 4-bit sub-CPU 7508 holds port 23 (pin 5) high. This causes IC 12D
to hold its pin 11 low, putting transistors 024 and 032 in conduction. 024 feeds the battery
voltage (VB) to pin 16 of IC 1 D (power terminal pin) to enable the AD converter ,uPD7001. 032
feeds VB to the voltage divider (resistors R69 and R57). The divided voltage across R57 is fed
to the AN 1 channel input of the AD converter which converts the input voltage to a 6-bit digi-
tal value representing a voltage value from OV to 2.0V in a minimum increment of approximate-
ly 32 mV. When the digital value falls to D9H (approx. 1.7V) or below, the sub-CPU detects a
low voltage condition.
F1
VB
032 R69
~ ______________ ~r- __ - . __ - . ______
1-il ANi
R59
10K
120
H.eK
,uPD7001
(1 D)
16 VOO
~------------~~
Q
24
100K R112
L....-_ _ ........ _ _
--Q
VB
Fig.2-11 Battery Voltage Detection Circuit
2-17